MP3387A ― 6-STRINGS, MAX 80MA/STRING, STEP-UP, WHITE LED DRIVER

MP3387A Rev. 1.0

www.MonolithicPower.com

13

5/17/2017

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Selecting the Dimming Control Mode

The MP3387A provides two different dimming

methods: direct PWM dimming and combined

analog and PWM dimming.

1) Direct PWM Dimming

Pull MIX low to operate the MP3387A in direct

PWM dimming mode. Apply a PWM dimming

The output LED current is chopped following the

direct PWM dimming signal duty cycle and

frequency. The average output current is equal to

depends on the LED current dimming ratio. A

lower input PWM dimming frequency results in a

better LED dimming ratio. Usually, a 200Hz to

2kHz PWM signal is sufficient for most

applications.

2) Combined Analog and PWM Dimming

Pull MIX high to operate the MP3387A in

combined analog and PWM dimming mode with

25% as its transfer point. In default, MIX is pulled

high. A PWM signal is applied to PWMI. When

the duty cycle of the PWM dimming signal is

≥25%, the IC dims the LED current with analog

mode, and the LED current amplitude is equal to

cycle of PWM dimming signal). Analog dimming

helps reduce LED power loss and prevent

audible noise issues. When the duty cycle of the

PWM dimming signal is <25%, the IC dims the

LED current in PWM mode. The LED current

The actual LED current dimming frequency is

switched automatically from 100Hz ~ 20kHz to

24kHz ~ 27.5kHz of the input PWM dimming

signal frequency. By doing this, the MP3387A

can improve the low LED current dimming ratio

and prevent troublesome audible noise issues.

PCB Layout Guidelines

Careful attention must be paid to the PCB board

layout and component placement. Proper layout

of the high frequency switching path is critical to

prevent noise and electromagnetic interference

problems. For best results, refer to Figure 2 and

follow the guidelines below.

1.

Keep the loop of LX to PGND (U1), the output

diode (D1), and the output capacitor (C2) as

short as possible, since it is flowing with high

frequency pulse current.

2.

Connect the IC’s exposed pad to GND

internally.

3. Refer all logic signals to GND.

4.

Connect PGND to GND externally.

5.

Keep PGND away from the logic signals.

Figure 2: Recommended Layout

LX

PGND

GND

D1

C2

R1

Iset

Rset

U1

R2